U.S. patent number 11,424,645 [Application Number 16/097,656] was granted by the patent office on 2022-08-23 for foreign object detection in a wireless power transfer system.
This patent grant is currently assigned to Koninklijke Philips N.V.. The grantee listed for this patent is KONINKLIJKE PHILIPS N.V.. Invention is credited to Wilhelmus Gerardus Maria Ettes, Petrus Carolus Maria Frissen, Aditya Mehendale, Willem Potze, Antonius Adriaan Maria Staring, Andries Van Wageningen.
United States Patent |
11,424,645 |
Van Wageningen , et
al. |
August 23, 2022 |
Foreign object detection in a wireless power transfer system
Abstract
A wireless power transfer system includes a power receiver (105)
receiving a power transfer from a power transmitter (101) via a
wireless inductive power transfer signal. The power transmitter
(101) comprises a transmit power coil (103) generating the power
transfer signal. A test signal coil (209) coupled to a test signal
generator (211) generates a magnetic test signal. A plurality of
spatially distributed detection coils (213) is coupled to
measurement unit (215) generating a set of measurement values
reflecting signals induced in the detection coils (213) by the
magnetic test signal. A processor (217) determines a measurement
spatial distribution of the measurement value where the spatial
distribution reflects positions of the detection coils (213). A
foreign object detector (219) detects a presence of a foreign
object in response to a comparison of the measurement spatial
distribution to a reference spatial distribution. The foreign
object detector (219) is arranged to determine the reference
spatial distribution in response to data received from the power
receiving device (105).
Inventors: |
Van Wageningen; Andries
(Wijlre, NL), Staring; Antonius Adriaan Maria
(Eindhoven, NL), Ettes; Wilhelmus Gerardus Maria
(Leeuwarden, NL), Frissen; Petrus Carolus Maria
(Beek, NL), Mehendale; Aditya (Geldrop,
NL), Potze; Willem (Geldrop, NL) |
Applicant: |
Name |
City |
State |
Country |
Type |
KONINKLIJKE PHILIPS N.V. |
Eindhoven |
N/A |
NL |
|
|
Assignee: |
Koninklijke Philips N.V.
(Eindhoven, NL)
|
Family
ID: |
1000006515944 |
Appl.
No.: |
16/097,656 |
Filed: |
May 2, 2017 |
PCT
Filed: |
May 02, 2017 |
PCT No.: |
PCT/EP2017/060319 |
371(c)(1),(2),(4) Date: |
October 30, 2018 |
PCT
Pub. No.: |
WO2017/194338 |
PCT
Pub. Date: |
November 16, 2017 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20200328616 A1 |
Oct 15, 2020 |
|
Foreign Application Priority Data
|
|
|
|
|
May 10, 2016 [EP] |
|
|
16168995 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02J
50/60 (20160201); G01R 31/42 (20130101); H02J
50/12 (20160201); G01R 27/2611 (20130101) |
Current International
Class: |
H02J
50/60 (20160101); H02J 50/12 (20160101); G01R
27/26 (20060101); G01R 31/42 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
202009009693 |
|
Dec 2010 |
|
DE |
|
102014207427 |
|
Oct 2015 |
|
DE |
|
2879140 |
|
Jun 2015 |
|
EP |
|
2508923 |
|
Jun 2014 |
|
GB |
|
02855929 |
|
Feb 1999 |
|
JP |
|
2011120425 |
|
Jun 2011 |
|
JP |
|
2013062180 |
|
Apr 2013 |
|
JP |
|
2015159690 |
|
Sep 2015 |
|
JP |
|
2016054596 |
|
Apr 2016 |
|
JP |
|
2007107888 |
|
Sep 2007 |
|
WO |
|
2014129181 |
|
Aug 2014 |
|
WO |
|
2015165703 |
|
Nov 2015 |
|
WO |
|
2016010493 |
|
Jan 2016 |
|
WO |
|
Other References
Wireless Power Consortium, Accessed Oct. 30, 2018,
https://www.wirelesspowerconsortium.com/index.html. cited by
applicant.
|
Primary Examiner: Cavallari; Daniel
Claims
The invention claimed is:
1. A power transmitter for a wireless power transfer system
including a power receiver for receiving a power transfer from the
power transmitter via a wireless inductive power transfer signal;
the power transmitter comprising: a power output circuit comprising
a transmit power coil for generating the wireless inductive power
transfer signal; a test signal coil for generating a magnetic test
signal; a test signal generator coupled to the test signal coil and
arranged to feed a test signal to the test signal coil resulting in
the generation of the magnetic test signal; a plurality of
spatially distributed detection coils; a measurement unit for
generating a set of measurement values reflecting signals induced
in the spatially distributed detection coils by the magnetic test
signal; a processor for determining a measurement spatial
distribution of the measurement values, the spatial distribution
reflecting positions of the detection coils; and a foreign object
detector arranged to detect a presence of a foreign object in
response to a comparison of the measurement spatial distribution to
a reference spatial distribution, wherein the foreign object
detector is arranged to determine the reference spatial
distribution in response to data received from the power receiving
device.
2. A power transmitter for a wireless power transfer system
including a power receiver for receiving a power transfer from the
power transmitter via a wireless inductive power transfer signal;
the power transmitter comprising: a power output circuit comprising
a transmit power coil for generating the wireless inductive power
transfer signal; a test signal coil for generating a magnetic test
signal; a test signal generator coupled to the test signal coil and
arranged to feed a test signal to the test signal coil resulting in
the generation of the magnetic test signal; a plurality of
spatially distributed detection coils; a measurement unit for
generating a set of measurement values reflecting signals induced
in the spatially distributed detection coils by the magnetic test
signal; a processor for determining a measurement spatial
distribution of the measurement values, the spatial distribution
reflecting positions of the detection coils; and a foreign object
detector arranged to detect a presence of a foreign object in
response to a comparison of the measurement spatial distribution to
a reference spatial distribution, wherein the foreign object
detector is arranged to geometrically align the measurement spatial
distribution and the reference spatial distribution by performing a
geometric transformation of at least one of the measurement spatial
distribution and the reference spatial distribution prior to the
comparison.
3. The power transmitter of claim 1 wherein the reference spatial
distribution represents a scenario with no foreign object
present.
4. The power transmitter of claim 1 wherein the reference spatial
distribution represents a scenario with a power receiving device
present.
5. The power transmitter of claim 1 wherein the foreign object
detector is arranged to store a copy of the measurement spatial
distribution, and to use the stored measurement spatial
distribution as the reference spatial distribution for future
comparisons.
6. The power transmitter of claim 2 wherein the geometric
transformation includes at least one of a translation and a
rotation.
7. The power transmitter of claim 1 further comprising a user
output unit, and wherein the foreign object detector is arranged to
generate a user output of an indication of a parameter of the
geometric transformation.
8. The power transmitter of claim 1 wherein the measurement unit is
arranged to generate a plurality of sets of measurement values for
different frequencies of the magnetic test signal, the processor is
arranged to generate plurality of measurement spatial distributions
by generating a measurement spatial distribution for each set of
measurement values, and the foreign object detector is arranged to
detect the presence of the foreign object in response to
comparisons of the plurality of measurement spatial distributions
to at least one reference spatial distribution.
9. The power transmitter of claim 1 wherein at least some of the
plurality of spatially distributed detection coils has a maximum
dimension not exceeding 20 mm and the plurality of spatially
distributed detection coils comprises no less than 20 detection
coils.
10. The power transmitter of claim 1 wherein the measurement unit
is arranged to generate the set of measurement values by
sequentially measuring induced signals of the plurality of
spatially distributed detection coils, the sequential measuring
comprising separately measuring subsets of the plurality of
spatially distributed detection coils.
11. A wireless power transfer system comprising a power transmitter
and a power receiving device for receiving a power transfer from
the power transmitter via a wireless inductive power signal; the
power transmitter comprising: a power output circuit comprising a
transmit power coil for generating the wireless inductive power
transfer signal; a test signal coil for generating a magnetic test
signal; a test signal generator coupled to the test signal coil and
arranged to feed a test signal to the test signal coil resulting in
the generation of the magnetic test signal; a plurality of
spatially distributed detection coils; a measurement unit for
generating a set of measurement values reflecting signals induced
in the spatially distributed detection coils by the magnetic test
signal; a processor for determining a measurement spatial
distribution of the measurement values, the spatial distribution
reflecting positions of the spatially distributed detection coils;
and a foreign object detector arranged to detect a presence of a
foreign object in response to a comparison of the measurement
spatial distribution to a reference spatial distribution, wherein
the foreign object detector is arranged to geometrically align the
measurement spatial distribution and the reference spatial
distribution by performing a geometric transformation of at least
one of the measurement spatial distribution and the reference
spatial distribution prior to the comparison.
12. A wireless power transfer system comprising a power transmitter
and a power receiving device for receiving a power transfer from
the power transmitter via a wireless inductive power signal; the
power transmitter comprising: a power output circuit comprising a
transmit power coil for generating the wireless inductive power
transfer signal; a test signal coil for generating a magnetic test
signal; a test signal generator coupled to the test signal coil and
arranged to feed a test signal to the test signal coil resulting in
the generation of the magnetic test signal; a plurality of
spatially distributed detection coils; a measurement unit for
generating a set of measurement values reflecting signals induced
in the spatially distributed detection coils by the magnetic test
signal; a processor for determining a measurement spatial
distribution of the measurement values, the spatial distribution
reflecting positions of the spatially distributed detection coils;
and a foreign object detector arranged to detect a presence of a
foreign object in response to a comparison of the measurement
spatial distribution to a reference spatial distribution, wherein
the power receiving device is arranged to transmit the reference
spatial distribution to the power transmitter and the power
transmitter is arranged to receive the reference spatial
distribution from the power receiving device.
13. A method of operation for a wireless power transfer system
comprising a power transmitter and a power receiving device for
receiving a power transfer from the power transmitter via a
wireless inductive power signal; the power transmitter comprising:
a power output circuit comprising a transmit power coil for
generating the wireless inductive power transfer signal; a test
signal coil for generating a magnetic test signal; a plurality of
spatially distributed detection coils; and the method comprising
the power transmitter performing the steps of: feeding a test
signal to the test signal coil resulting in the generation of the
magnetic test signal; generating a set of measurement values
reflecting signals induced in the spatially distributed detection
coils by the magnetic field signal; determining a measurement
spatial distribution of the measurement values, the spatial
distribution reflecting positions of the detection coils; and
detecting a presence of a foreign object in response to a
comparison of the measurement spatial distribution to a reference
spatial distribution, wherein the comparison comprises
geometrically aligning the measurement spatial distribution and the
reference spatial distribution by performing a geometric
transformation of at least one of the measurement spatial
distribution and the reference spatial distribution prior to the
comparison.
14. A method of operation for a wireless power transfer system
comprising a power transmitter and a power receiving device for
receiving a power transfer from the power transmitter via a
wireless inductive power signal; the power transmitter comprising:
a power output circuit comprising a transmit power coil for
generating the wireless inductive power transfer signal; a test
signal coil for generating a magnetic test signal; a plurality of
spatially distributed detection coils; and the method comprising
the power transmitter performing the steps of: feeding a test
signal to the test signal coil resulting in the generation of the
magnetic test signal; generating a set of measurement values
reflecting signals induced in the spatially distributed detection
coils by the magnetic field signal; determining a measurement
spatial distribution of the measurement values, the spatial
distribution reflecting positions of the detection coils; and
detecting a presence of a foreign object in response to a
comparison of the measurement spatial distribution to a reference
spatial distribution, and further comprising the steps of the power
receiving device transmitting the reference spatial distribution to
the power transmitter and the power transmitter receiving the
reference spatial distribution from the power receiving device.
15. A method of operation for a power transmitter of a wireless
power transfer system also comprising a power receiving device for
receiving a power transfer from the power transmitter via a
wireless inductive power signal; the power transmitter comprising:
a power output circuit comprising a transmit power coil for
generating the wireless inductive power transfer signal; a test
signal coil for generating a magnetic test signal; a test signal
generator coupled to the test signal coil and arranged to feed a
test signal to the test signal coil resulting in the generation of
the magnetic test signal; a plurality of spatially distributed
detection coils; a measurement unit for generating a set of
measurement values reflecting signals induced in the spatially
distributed detection coils by the magnetic test signal; a
processor for determining a measurement spatial distribution of the
measurement values, the spatial distribution reflecting positions
of the detection coils; and a foreign object detector arranged to
detect a presence of a foreign object in response to a comparison
of the measurement spatial distribution to a reference spatial
distribution, wherein the comparison comprises geometrically
aligning the measurement spatial distribution and the reference
spatial distribution by performing a geometric transformation of at
least one of the measurement spatial distribution and the reference
spatial distribution prior to the comparison.
Description
CROSS-REFERENCE TO PRIOR APPLICATIONS
This application is the U.S. National Phase application under 35
U.S.C. .sctn. 371 of International Application No.
PCT/EP2017/060319, filed on 2 May 2017, which claims the benefit of
European Patent Application No. 16168995.5, filed on 10 May 2016.
These applications are hereby incorporated by reference herein.
FIELD OF THE INVENTION
The invention relates to foreign object detection in an inductive
power transfer system and in particular, but not exclusively, to
foreign object detection for a power transmitter providing
inductive power transfer using elements compatible with the Qi
Specifications for wireless power transfer systems.
BACKGROUND OF THE INVENTION
Most present day systems require a dedicated electrical contact in
order to be powered from an external power supply. However, this
tends to be impractical and requires the user to physically insert
connectors or otherwise establish a physical electrical contact.
Typically, power requirements also differ significantly, and
currently most devices are provided with their own dedicated power
supply resulting in a typical user having a large number of
different power supplies with each power supply being dedicated to
a specific device. Although, the use of internal batteries may
avoid the need for a wired connection to a power supply during use,
this only provides a partial solution as the batteries will need
recharging (or replacing). The use of batteries may also add
substantially to the weight and potentially cost and size of the
devices.
In order to provide a significantly improved user experience, it
has been proposed to use a wireless power supply wherein power is
inductively transferred from a transmitter inductor in a power
transmitter device to a receiver coil in the individual
devices.
Power transmission via magnetic induction is a well-known concept,
mostly applied in transformers having a tight coupling between a
primary transmitter inductor and a secondary receiver coil. By
separating the primary transmitter inductor and the secondary
receiver coil between two devices, wireless power transfer between
these becomes possible based on the principle of a loosely coupled
transformer.
Such an arrangement allows a wireless power transfer to the device
without requiring any wires or physical electrical connections to
be made. Indeed, it may simply allow a device to be placed adjacent
to, or on top of, the transmitter inductor in order to be recharged
or powered externally. For example, power transmitter devices may
be arranged with a horizontal surface on which a device can simply
be placed in order to be powered.
Furthermore, such wireless power transfer arrangements may
advantageously be designed such that the power transmitter device
can be used with a range of power receiver devices. In particular,
a wireless power transfer approach, known as the Qi Specifications,
has been defined and is currently being developed further. This
approach allows power transmitter devices that meet the Qi
Specifications to be used with power receiver devices that also
meet the Qi Specifications without these having to be from the same
manufacturer or having to be dedicated to each other. The Qi
standard further includes some functionality for allowing the
operation to be adapted to the specific power receiver device (e.g.
dependent on the specific power drain).
The Qi Specification is developed by the Wireless Power Consortium
and more information can e.g. be found on their website:
http://www.wirelesspowerconsortium.com/index.html, where in
particular the defined Specification documents can be found.
A potential problem with wireless power transfer is that power may
unintentionally be transferred to e.g. metallic objects that happen
to be in the vicinity of the power transmitter. For example, if a
foreign object, such as e.g. a coin, key, ring etc., is placed upon
the power transmitter platform arranged to receive a power
receiver, the magnetic flux generated by the transmitter coil will
introduce eddy currents in the metal objects which will cause the
objects to heat up. The heat increase may be very significant and
may be highly disadvantageous.
In order to reduce the risk of such scenarios arising, it has been
proposed to introduce foreign object detection where the power
transmitter can detect the presence of a foreign object and reduce
the transmit power and/or generate a user alert when a positive
detection occurs. For example, the Qi system includes functionality
for detecting a foreign object, and for reducing power if a foreign
object is detected. Specifically, Qi specification version 1.2.1,
section 11 describes various methods of detecting a foreign
object.
One method to detect such foreign objects is by determining
unaccounted for power losses, as e.g. disclosed in WO 2012127335.
Both the power receiver and the power transmitter measure their
power, and the receiver communicates its measured received power to
the power transmitter. When the power transmitter detects a
significant difference between the power sent by the transmitter
and the power received by the receiver, an unwanted foreign object
may potentially be present, and the power transfer may be reduced
or aborted for safety reasons. This power loss method requires
synchronized accurate power measurements performed by the power
transmitter and the power receiver.
For example, in the Qi power transfer standard, the power receiver
estimates its received power e.g. by measuring the rectified
voltage and current, multiplying them and adding an estimate of the
internal power losses in the power receiver (e.g. losses of the
rectifier, the receive coil, metal parts being part of the receiver
etc.). The power receiver reports the determined received power to
the power transmitter with a minimum rate of e.g. every four
seconds.
The power transmitter estimates its transmitted power, e.g. by
measuring the DC input voltage and current of the inverter,
multiplying them and correcting the result by subtracting an
estimation of the internal power losses in the transmitter, such as
e.g. the estimated power loss in the inverter, the primary coil and
metal parts that are part of the power transmitter.
The power transmitter can estimate the power loss by subtracting
the reported received power from the transmitted power. If the
difference exceeds a threshold, the transmitter will assume that
too much power is dissipated in a foreign object, and it can then
proceed to terminate the power transfer.
Alternatively, it has been proposed to measure the quality or
Q-factor of the resonant circuit formed by the primary and
secondary coils together with the corresponding capacitances and
resistances. A reduction in the measured Q-factor may be indicative
of a foreign object being present.
In practice, it tends to be difficult to achieve sufficient
detection accuracy using the methods described in the Qi
specification. This difficulty is exacerbated by a number of
uncertainties about the specific current operating conditions.
For example, a particular problem is the potential presence of
friendly metals (i.e. metal parts of the device that contains the
power receiver) as the magnetic and electrical properties of these
may be unknown and therefore may be difficult to compensate for.
Moreover, the spatial alignment between the power receiver and the
power transmitter is typically not known and this may substantially
affect the measured values. Also, the lateral extension of the
generated magnetic field is typically not known, and may vary
substantially between different power transmitters.
Further, undesirable heating may result from even relatively small
amounts of power being dissipated in a metallic foreign object.
Therefore, it is necessary to detect even a small power discrepancy
between the transmitted and received power and this may be
particularly difficult when the power levels of the power transfer
increase.
The power estimates of both the power transmitter and the power
receiver may typically have an accuracy of around .+-.1%. For a 15
W power receiver load, the received power is typically 16 W or more
and therefore typical power estimates may in such an example have
an uncertainty of around .+-.160 mW. Accordingly, the estimated
power difference/loss will have an uncertainty of .+-.320 mW,
spanning a range of 640 mW. Determining whether the true power loss
is less than 500 mW thus becomes a difficult task.
The Q factor degradation approach may in many scenarios have a
better sensitivity for detecting the presence of metal objects.
However, it may still not provide sufficient accuracy and e.g. may
also suffer from the influence of friendly metal.
The problems increase for increasing power levels and therefore the
existing approaches may be less suitable for the higher power
levels that are envisaged in the future.
Specifically, the uncertainty of the power loss estimate may
increase substantially for higher power levels. However, detection
is still required at the same absolute power levels (e.g. it may be
required that a power loss of 500 mW (or less) in a foreign object
must be detected).
Another issue is that higher power levels tend to require
physically larger coils in order to effectively transfer the higher
levels of power. For example, the coils in a Qi system for power
levels of around 1 W . . . 30 W tend to have e.g. a diameter of
approximately 20 mm . . . 60 mm whereas the coils in systems for
power levels in the 200 W . . . 2 kW ranges will have a typical
diameter of approximately 100 mm . . . 2500 mm. However, such
larger coils impact the detection performance of both the power
difference and Q degradation approaches. This is particularly the
case for detection of smaller foreign objects, such as for example
for detection of a small coin.
Another issue is that the power receiver may in some cases not be
able to provide information on the received power, or may have
characteristics that differentiate substantially from the nominal
or expected power receiver. For example, the power receiver may be
a power receiving device that uses the magnetic field for direct
inductive heating. This would for example be the case for a device
that heats up a metal plate by eddy currents induced by the power
transfer signal e.g. in order to cook water or food. The friendly
metal of such a device has an enormous influence on the
measurements and at first glance makes it almost impossible to
distinguish a foreign metal object from the friendly metal. Also,
such power receivers may be very simple and may not comprise
functionality for effectively measuring and reporting the received
power to the power receiver.
Accordingly, current algorithms tend to be suboptimal and may in
some scenarios and examples provide less than optimum performance.
In particular, they may result in foreign objects that are present
not being detected, or in false detections of foreign objects when
none are present.
Hence, an improved object detection would be advantageous and in
particular an approach allowing increased flexibility, reduced
cost, reduced complexity, improved object detection, fewer false
detections and missed detections, and/or improved performance would
be advantageous.
SUMMARY OF THE INVENTION
Accordingly, the Invention seeks to preferably mitigate, alleviate
or eliminate one or more of the above mentioned disadvantages
singly or in any combination. According to an aspect of the
invention there is provided a power transmitter for a wireless
power transfer system including a power receiver for receiving a
power transfer from the power transmitter via a wireless inductive
power transfer signal; the power transmitter comprising: a power
output circuit comprising a transmit power coil for generating the
wireless inductive power transfer signal; a test signal coil for
generating a magnetic test signal; a test signal generator coupled
to the test signal coil and arranged to feed a test signal to the
test signal coil resulting in the generation of the magnetic test
signal; a plurality of spatially distributed detection coils; a
measurement unit for generating a set of measurement values
reflecting signals induced in the spatially distributed detection
coils by the magnetic test signal; a processor for determining a
measurement spatial distribution of the measurement values, the
spatial distribution reflecting positions of the detection coils;
and a foreign object detector arranged to detect a presence of a
foreign object in response to a comparison of the measurement
spatial distribution to a reference spatial distribution, wherein
the foreign object detector (219) is arranged to determine the
reference spatial distribution in response to data received from
the power receiving device (105).
According to another aspect of the invention there is provided a
power transmitter for a wireless power transfer system including a
power receiver for receiving a power transfer from the power
transmitter via a wireless inductive power transfer signal; the
power transmitter comprising: a power output circuit comprising a
transmit power coil for generating the wireless inductive power
transfer signal; a test signal coil for generating a magnetic test
signal; a test signal generator coupled to the test signal coil and
arranged to feed a test signal to the test signal coil resulting in
the generation of the magnetic test signal; a plurality of
spatially distributed detection coils; a measurement unit for
generating a set of measurement values reflecting signals induced
in the spatially distributed detection coils by the magnetic test
signal; a processor for determining a measurement spatial
distribution of the measurement values, the spatial distribution
reflecting positions of the detection coils; and a foreign object
detector arranged to detect a presence of a foreign object in
response to a comparison of the measurement spatial distribution to
a reference spatial distribution, wherein the foreign object
detector is arranged to geometrically align the measurement spatial
distribution and the reference spatial distribution by performing a
geometric transformation of at least one of the measurement spatial
distribution and the reference spatial distribution prior to the
comparison.
The invention may provide improved foreign object detection in many
embodiments and scenarios. In particular, the approach may in many
embodiments reduce the risk of false detections and/or reduce the
risk of missed detections of a foreign object.
A particular advantage of the approach is that it may in many
embodiments and scenarios allow an efficient foreign object
detection to be performed even when the power receiving device is
present. The approach may in many embodiments allow a
differentiation to be made between friendly metal (typically being
part of the power receiving device and thus intended to be present)
and foreign metal of a foreign object (not part of the power
receiving device). This may in many scenarios provide a
substantially more effective and reliable foreign object and/or may
allow an improved user experience to be provided.
The geometric alignment may substantially increase detection
accuracy in many embodiments. It may provide increased flexibility
and may in many embodiments allow the power transmitter to adapt to
the current scenario and configuration. The approach may in many
embodiments provide increased freedom in the placement of power
receivers. For example, it may allow a user to position a power
receiving device, such as e.g. a kitchen appliance, anywhere within
a given area and orientated in any way.
The spatial distribution(s) of measurement values may comprise data
indicative of both measured values and a spatial position of the
measurements. The position indication may be a relative position
indication, e.g. indicating a position relative to other
measurement values. The spatial position data may be indicated by a
position or arrangement of the measurement values relative to each
other. For example, a spatial distribution may be represented by a
data structure of measurement values where the location of
measurement values in the data structure represents spatial
position information for the measurement values. Specifically, the
relative order of measurement values in the data structure may
correspond to the spatial relationship between the detection coils
corresponding to the measurement values.
The detection coils may specifically be arranged in a planar array.
The planar array may cover a power transfer surface of the power
transmitter arranged to receive the power receiving device.
The test signal coil and the transmit power coil may be the same
coil, i.e. the transmit power coil may also be used as the test
signal coil.
In accordance with an optional feature of the invention, the
reference spatial distribution represents a scenario with no
foreign object present.
This may provide improved foreign object detection in many
embodiments.
In many embodiments, the reference spatial distribution may
represent a scenario where no objects are present.
In accordance with an optional feature of the invention, the
reference spatial distribution represents a scenario with a power
receiving device present.
This may provide improved foreign object detection in many
embodiments. The approach may in particular in many embodiments
enable, facilitate, or improve foreign object detection while the
power receiving device is present
In accordance with an optional feature of the invention, the
foreign object detector is arranged to store a copy of the
measurement spatial distribution, and to use the stored measurement
spatial distribution as the reference spatial distribution for
future comparisons.
This may facilitate and/or improve foreign object detection. In
particular, it may provide an efficient approach for generating a
reliable reference spatial distribution suitable for foreign object
detection and customized to the specific power transmitter and
possibly power receiving device.
The foreign object detector may specifically be arranged to store
the measurement spatial distribution in response to a user input
indication that no foreign object is present, or e.g. in many
embodiments indicating that the power receiving device is present
but no foreign object is present.
In accordance with an aspect of the invention, the foreign object
detector is arranged to determine the reference spatial
distribution in response to data received from the power receiving
device.
This may provide a more flexible approach while allowing relatively
low complexity of the power transmitter. For example, it may
provide foreign object detection specifically customized for the
specific power receiving device without requiring the
characteristics of this to necessarily be known in advance by the
power transmitter, and specifically without requiring a reference
spatial distribution to be stored by the power transmitter for the
power receiving device.
The foreign object detector is arranged to geometrically align the
measurement spatial distribution and the reference spatial
distribution by performing a geometric transformation of at least
one of the measurement spatial distribution and the reference
spatial distribution prior to the comparison.
This may improve foreign object detection in many embodiments and
may in particular provide improved adaptation of the detection to
the current conditions.
In accordance with an optional feature of the invention, the
geometric transformation may include a translation and/or rotation.
The foreign object detector may specifically be arranged to align
the measurement spatial distribution and the reference spatial
distribution such that a match between them is maximized
(corresponding to a minimization of a difference measure).
In accordance with an optional feature of the invention, the power
transmitter further comprises a user output unit, and wherein the
foreign object detector is arranged to generate a user output of an
indication of a parameter of the geometric transformation.
This may for example provide an indication to the user allowing the
user to move the power receiving device to a more optimal position
thereby improving the power transfer operation.
In accordance with an optional feature of the invention, the
measurement unit is arranged to generate a plurality of sets of
measurement values for different frequencies of the magnetic test
signal, the processor is arranged to generate plurality of
measurement spatial distributions by generating a measurement
spatial distribution for each set of measurement values, and the
foreign object detector is arranged to detect the presence of the
foreign object in response to comparisons of the plurality of
measurement spatial distributions to at least one reference spatial
distribution.
This may allow a more accurate foreign object detection in many
scenarios.
In some embodiments, the test signal generator is arranged to
generate a plurality of test signals with different frequencies
resulting in a plurality of magnetic test signals with different
frequencies; the measurement unit is arranged to generate a
plurality of sets of measurement values, each set of measurement
values corresponding to one test signal; the processing is arranged
to generate a plurality of measurement spatial distributions by
generating a measurement spatial distribution for each of the sets
of measurement values; and the foreign object detector is arranged
to detect the response of the foreign object in response to a
plurality of comparisons, each comparison being between a
measurement spatial distribution of the plurality of measurement
spatial distributions and a reference spatial distribution.
In accordance with an optional feature of the invention, at least
some of the plurality of spatially distributed detection coils has
a maximum dimension not exceeding 20 mm and the plurality of
spatially distributed detection coils comprises no less than 20
detection coils.
This may provide particularly advantageous operation in many
embodiments, and may specifically provide an advantageous trade-off
between complexity and detection performance.
In accordance with an optional feature of the invention, the
measurement unit is arranged to generate the set of measurement
values by sequentially measuring induced signals of the plurality
of spatially distributed detection coils, the sequential measuring
comprising separately measuring subsets of the plurality of
spatially distributed detection coils.
This may in many applications reduce complexity of the power
transmitter.
According to an aspect of the invention there is provided wireless
power transfer system comprising a power transmitter and a power
receiving device for receiving a power transfer from the power
transmitter via a wireless inductive power signal; the power
transmitter comprising: a power output circuit comprising a
transmit power coil for generating the wireless inductive power
transfer signal; a test signal coil for generating a magnetic test
signal; a test signal generator coupled to the test signal coil and
arranged to feed a test signal to the test signal coil resulting in
the generation of the magnetic test signal; a plurality of
spatially distributed detection coils; a measurement unit for
generating a set of measurement values reflecting signals induced
in the spatially distributed detection coils by the magnetic test
signal; a processor for determining a measurement spatial
distribution of the measurement values, the spatial distribution
reflecting positions of the spatially distributed detection coils;
and a foreign object detector arranged to detect a presence of a
foreign object in response to a comparison of the measurement
spatial distribution to a reference spatial distribution, wherein
the foreign object detector is arranged to geometrically align the
measurement spatial distribution and the reference spatial
distribution by performing a geometric transformation of at least
one of the measurement spatial distribution and the reference
spatial distribution prior to the comparison.
In accordance with an optional feature of the invention, the power
receiving device is arranged to transmit the reference spatial
distribution to the power transmitter and the power transmitter is
arranged to receive the reference spatial distribution from the
power receiving device.
According to an aspect of the invention there is provided a method
of operation for a wireless power transfer system comprising a
power transmitter and a power receiving device for receiving a
power transfer from the power transmitter via a wireless inductive
power signal; the power transmitter comprising: a power output
circuit comprising a transmit power coil for generating the
wireless inductive power transfer signal; a test signal coil for
generating a magnetic test signal; a plurality of spatially
distributed detection coils; and the method comprising the power
transmitter performing the steps of: feeding a test signal to the
test signal coil resulting in the generation of the magnetic test
signal; generating a set of measurement values reflecting signals
induced in the spatially distributed detection coils by the
magnetic field signal; determining a measurement spatial
distribution of the measurement values, the spatial distribution
reflecting positions of the detection coils; and detecting a
presence of a foreign object in response to a comparison of the
measurement spatial distribution to a reference spatial
distribution, wherein the comparison comprises geometrically
aligning the measurement spatial distribution and the reference
spatial distribution by performing a geometric transformation of at
least one of the measurement spatial distribution and the reference
spatial distribution prior to the comparison.
In accordance with an optional feature of the invention, the method
further comprises the steps of the power receiving device
transmitting the reference spatial distribution to the power
transmitter and the power transmitter receiving the reference
spatial distribution from the power receiving device.
According to an aspect of the invention there is provided a method
of operation for a power transmitter of a wireless power transfer
system also comprising a power receiving device for receiving a
power transfer from the power transmitter via a wireless inductive
power signal; the power transmitter comprising: a power output
circuit comprising a transmit power coil for generating the
wireless inductive power transfer signal; a test signal coil for
generating a magnetic test signal; a test signal generator coupled
to the test signal coil and arranged to feed a test signal to the
test signal coil resulting in the generation of the magnetic test
signal; a plurality of spatially distributed detection coils; a
measurement unit for generating a set of measurement values
reflecting signals induced in the spatially distributed detection
coils by the magnetic test signal; a processor for determining a
measurement spatial distribution of the measurement values, the
spatial distribution reflecting positions of the detection coils;
and a foreign object detector arranged to detect a presence of a
foreign object in response to a comparison of the measurement
spatial distribution to a reference spatial distribution, wherein
the comparison comprises geometrically aligning the measurement
spatial distribution and the reference spatial distribution by
performing a geometric transformation of at least one of the
measurement spatial distribution and the reference spatial
distribution prior to the comparison.
These and other aspects, features and advantages of the invention
will be apparent from and elucidated with reference to the
embodiment(s) described hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will be described, by way of example
only, with reference to the drawings, in which
FIG. 1 illustrates an example of elements of a power transfer
system in accordance with some embodiments of the invention;
FIG. 2 illustrates an example of elements of a power transfer
system in accordance with some embodiments of the invention;
FIG. 3 illustrates an example of an arrangement of coils in a power
transmitter in accordance with some embodiments of the
invention;
FIG. 4 illustrates an example of a cross section of a coil
arrangement of a power transmitter in accordance with some
embodiments of the invention;
FIG. 5 illustrates an example of a magnetic field of the power
transfer system of FIG. 2 in the presence of two metallic objects;
and
FIG. 6 illustrates an example of elements of a power transfer
system in accordance with some embodiments of the invention.
DETAILED DESCRIPTION OF SOME EMBODIMENTS OF THE INVENTION
The following description focuses on embodiments of the invention
applicable to a wireless power transfer system utilizing a power
transfer approach such as known from the Qi specification. However,
it will be appreciated that the invention is not limited to this
application but may be applied to many other wireless power
transfer systems.
FIG. 1 illustrates an example of a power transfer system in
accordance with some embodiments of the invention. The power
transfer system comprises a power transmitter 101 which includes
(or is coupled to) a transmitter coil/inductor 103. The system
further comprises a power receiving device 105 which includes (or
is coupled to) a receiver coil/inductor 107.
The system provides a wireless inductive power transfer from the
power transmitter 101 to the power receiving device 105.
Specifically, the power transmitter 101 generates a wireless
inductive power transfer signal (also referred to as a power
transfer signal, power transfer signal or an inductive power
transfer signal), which is propagated as a magnetic flux by the
transmitter coil or inductor 103. The power transfer signal may
typically have a frequency between around 20 kHz to around 500 kHz,
and often for Qi compatible systems typically in the range from 95
kHz to 205 kHz (or e.g. for high power kitchen applications, the
frequency may e.g. typically be in the range between 20 kHz to 80
kHz). The transmitter coil 103 and the power receiving coil 107 are
loosely coupled and thus the power receiving coil 107 picks up (at
least part of) the power transfer signal from the power transmitter
101. Thus, the power is transferred from the power transmitter 101
to the power receiving device 105 via a wireless inductive coupling
from the transmitter coil 103 to the power receiving coil 107. The
term power transfer signal is mainly used to refer to the inductive
signal/magnetic field between the transmitter coil 103 and the
power receiving coil 107 (the magnetic flux signal), but it will be
appreciated that by equivalence it may also be considered and used
as a reference to an electrical signal provided to the transmitter
coil 103 or picked up by the power receiving coil 107.
In the example, the power receiving device 105 is specifically a
power receiver which receives power via a receive coil 107.
However, in other embodiments, the power receiving device 105 may
comprise a metallic element, such as a metallic heating element, in
which case the power transfer signal induces eddy currents
resulting in a direct heating of the element.
The system is arranged to transfer substantial power levels, and
specifically the power transmitter may support power levels in
excess of 500 mW, 1 W, 5 W, or 50 W in many embodiments. For
example, for Qi corresponding applications, the power transfers may
typically be in the 1-5 W power range for low power applications,
and in excess of 100 W and up to more than 1000 W for high power
applications, such as e.g. kitchen applications.
In the following, the operation of the power transmitter 101 and
the power receiving device 105 will be described with specific
reference to an embodiment in accordance with the Qi Specification
(except for the herein described (or consequential) modifications
and enhancements) or suitable for the higher power kitchen
specification being developed by the Wireless Power Consortium. In
particular, the power transmitter 101 and the power receiving
device 105 may substantially be compatible with the Qi
Specification version 1.0, 1.1 or 1.2 (except for the herein
described (or consequential) modifications and enhancements).
In wireless power transfer systems, the presence of a foreign
object (typically a conductive element extracting power from the
power transfer signal and not being part of the power receiving
device 105, i.e. being an unintended, undesired, and/or interfering
element to the power transfer) may be highly disadvantageous during
a power transfer. Such an undesired object is in the field known as
a foreign object.
A foreign object may not only reduce efficiency by adding a power
loss to the operation but may also degrade the power transfer
operation itself (e.g. by interfering with the power transfer
efficiency or extracting power not directly controlled e.g. by the
power transfer loop). In addition, the induction of currents in the
foreign object (specifically eddy currents in the metal part of a
foreign object) may result in an often highly undesirable heating
of the foreign object.
In order to address such scenarios, wireless power transfer systems
such as Qi include functionality for foreign object detection.
Specifically, the power transmitter comprises functionality seeking
to detect whether a foreign object is present. If so, the power
transmitter may e.g. terminate the power transfer or reduce the
maximum amount of power that can be transferred.
Current approaches proposed by the Qi Specifications are based on
detecting a power loss (by comparing the transmitted and the
reported received power) or detecting degradations in the quality Q
of the output resonance circuit. However, these approaches have
been found to provide suboptimal performance in many scenarios, and
they may specifically lead to inaccurate detection resulting in
missed detections and/or false positives where a foreign object is
detected despite no such object being present.
The system of FIG. 1 uses a different approach for foreign object
detection. The approach is based on using a plurality of spatially
distributed detection coils and generating a spatial distribution
of measurements of signals induced in these detection coils by a
magnetic test signal generated by a test signal being fed to a test
signal coil. The spatial distribution may in many embodiments be
perceived as a magnetic image and may reflect the objects in the
neighborhood that affect the magnetic field generated by the test
signal coil.
As an example, the approach may perform foreign object detection
based on using a separate test signal coil to generate a magnetic
test signal and thus a magnetic test field, with an array of small
detection coils measuring the influence of metal objects on the
field generated by the test signal coil. The detection coils may
typically be arranged in a matrix arrangement. The measurements may
specifically be suitably timed signal level measurements for each
of the detection coils. The spatial distribution of such
measurements can be considered to correspond to a magnetic image
with each detection coil representing a pixel of the image. Thus,
the spatial distribution may be a two dimensional arrangement of
the measurement values, and specifically may be a two dimensional
array or matrix of measurement values from the different detection
coils. Specifically, the spatial distribution of the measurement
values will reflect the positions of the detection coils. In this
way, the spatial distribution reflects the position at which the
individual measurements have been generated and thus represents the
spatial distribution or variation of the magnetic test field (such
as specifically the strength of the magnetic field resulting
(directly or indirectly) from the test signal being fed to the test
signal coil).
Thus, the exemplary approach can be viewed as the power transmitter
including coils that implement a magnetic camera generating a
magnetic image by each detection coil corresponding to a pixel.
In the example, the test signal coil 209 and the transmitter power
coil 103 are illustrated and described as separate and independent
coils. This may allow individual optimization of the individual
coils for the specific purpose. However, in other embodiments, the
test signal coil 209 and the transmitter power coil 103 may be the
same coil, i.e. the same coil may be used to generate the power
transfer signal and to generate the magnetic test signal. For
example, the two signals may be separated in the time domain using
a time division approach or may alternatively be separated e.g. in
the frequency domain by the signals having different frequencies.
This may allow a lower cost implementation in some scenarios.
The spatial distribution is compared to a reference spatial
distribution which reflects a scenario when no foreign object is
present, but possible with a power receiving device 105 being
present, i.e. it is compared to a reference magnetic image
corresponding to a situation with no foreign object being present.
If the generated magnetic image is sufficiently similar to the
reference magnetic image, the foreign object detector may decide
that no foreign object is present and otherwise it may decide that
the difference may be due to a foreign object potentially being
present.
For example, the power transmitter can detect the presence of a
metal object on its surface by taking a magnetic picture and
comparing it to the situation in which no object is present. If at
least one pixel shows a sufficiently large difference, a metal
object may be determined to be detected.
FIG. 2 illustrates an example of elements of the system of FIG. 1
in more detail.
The power transmitter 101 comprises a power output circuit
including a driver 201 and the transmitter coil 103. The driver 201
generates a power drive signal which is fed to the transmitter coil
103 resulting in this generating a magnetic/inductive power
transfer signal. The driver 201 is arranged to generate a drive
signal of sufficient power for the specific power transfer
operation. In some embodiments, the power range may be in the order
of a few tens of watts but in other embodiments it may be
substantially higher, and indeed may be more than a 1 kW.
The driver 201 may for example comprise an output stage in the form
of a full or half bridge switch arrangement as will be known to the
skilled person. It will also be appreciated that whereas FIG. 2
illustrates the driver 201 directly driving the transmitter coil
103, the transmitter coil 103 will typically be part of a resonance
circuit further comprising a resonance capacitor in series or
parallel with the transmitter coil 103.
The driver 201 is coupled to a power transmitter controller 203
which is arranged to control the operation of the power transmitter
101 and specifically to control the driver 201 to provide a
suitable drive signal (e.g. with the required power level). In
addition, the power transmitter controller 203 is arranged to
control the power transmitter 101 to perform the necessary and
desired functions for the power transmitter 101 to operate in
accordance with the appropriate specification or standards. In the
specific example, the power transmitter controller 203 may control
the power transmitter 101 to operate in accordance with the Qi
specifications (except for changes described herein).
The power transmitter 101 can thus support wireless power transfers
by generating a power transfer signal from which a power receiving
device can extract power. In the example of FIG. 2, the power
receiving device 105 is shown comprising a power receiving coil 107
coupled to a power receiver circuit 205. The power receiving
circuit 205 is arranged to receive the induced signal from the
power receiving coil 107 and to extract power that can be fed to an
external or internal load. The power receiving circuit 205 may for
example include a rectifier circuit, a voltage stabilization
circuit etc. as is known in the art.
The power receiving circuit 205 is further coupled to a power
receiver controller 207 which is arranged to control the operation
of the power receiver 105 to perform the necessary and desired
functions for the power receiver 105 to operate in accordance with
the appropriate specification or standards. In the specific
example, the power receiver controller 207 may control the power
receiver 105 to operate in accordance with the Qi specifications
(except for changes described herein). Such operation may include
establishing and maintaining suitable communication with the power
transmitter 101, e.g. to support the establishment of a power
control loop etc.
It will be appreciated that whereas FIGS. 1 and 2, illustrate the
power extraction by a power receiving coil 107 coupled to a power
receiving circuit 205, the power may in other embodiments be
extracted by other means. For example, in some embodiments, the
power receiving device 105 may comprise a metallic heating element
to which power is directly coupled by the induction of eddy
currents. In such examples, the power receiver controller 207 may
for example use a power control loop to control the transmitted
power level to result in a desired temperature.
The power transmitter 101 furthermore comprises functionality for
foreign object detection. The power transmitter 101 thus comprises
functionality for detecting if an object other than the power
receiving device 105 is present in the power transfer zone. Thus,
the foreign object detection may seek to detect a foreign object
being an object other than the power receiving device 105
extracting power from the power transfer signal.
The foreign object detection is based on the generation of a
magnetic test signal and accordingly the power transmitter 101
comprises a test signal coil 209 which is coupled to a test signal
generator 211. The test signal generator 211 generates an electric
test signal which is fed to the test signal coil 209 resulting in
the magnetic test signal being generated, i.e. the test signal
generator 211 generates a magnetic test field. In different
embodiments, the test signal from the test signal generator 211 may
e.g. be a short time interval of a periodic signal (e.g. a
sinewave, a square wave, a triangular wave signal etc.), a
continuous periodic signal, or may e.g. be a short pulse). A
corresponding time varying magnetic test signal is thus
generated.
The power transmitter 101 furthermore includes a plurality of
detection coils 213 which are arranged to detect the magnetic test
signal by this causing a voltage to be induced in the detection
coils 213. The detection coils are spatially distributed, and
specifically may be spatially distributed in a planar area. Thus,
the detection coils 213 may be distributed over an area which
specifically may be, or include, a power transfer area of the power
transmitter 101 for receiving the power receiving device 105.
The detection coils 213 are positioned relative to the test signal
coil 209 such that the generated magnetic test signal will induce a
voltage (and consequent current) in the detection coils 213, at
least in the case where no foreign object is present.
The detection coils 213 are coupled to a measurement unit 215 which
is arranged to perform measurements to generate a set of
measurement values reflecting signals induced in the spatially
distributed detection coils by the magnetic test signal. The
measurement unit 215 may specifically measure the current or
voltage of each of the detection coils 213 to generate a
measurement reflecting the level of the magnetic test signal at the
position of the corresponding detection coil 213.
As a specific example, each of the detection coils 213 may be
terminated by a high Ohmic resistor in the measurement unit 215,
and the measurement unit 215 may include an A/D converter which
samples the voltage across the resistor. The samples may be used to
generate a measurement reflecting e.g. the signal strength of the
magnetic test signal. In some embodiments, the measurement may e.g.
include an averaging, e.g. over at least one time period for the
magnetic test signal. In other embodiments, the measurement may
e.g. be a sample performed at an appropriate time. For example, the
sampling of the signal of the detection coils 213 may be with a
suitable phase offset relative to the test signal fed to the test
signal coil 209.
Thus, the measurement unit 215 generates a set of measurement
values which reflects the magnetic flux density of the magnetic
test signal at the different positions represented by the
individual detection coils 213. The detection coils 213 may
specifically be arranged in a planar array (or specifically matrix)
arrangement and the set of measurement values may accordingly
effectively be seen as a magnetic image where each pixel
corresponds to one of the detection coils 213.
The measurement unit 215 is coupled to a processor, henceforth
referred to as the spatial processor 217, which is fed the set of
measurements and which is arranged to determine a measurement
spatial distribution of the measurement values where the spatial
distribution reflects the positions of the detection coils 213.
The spatial distribution may for example be generated by arranging
the measurements in a data structure in accordance with the
physical spatial arrangement of the detection coils 213. For
example, a data structure may be used where measurements from
adjacent detection coils 213 are located next to each other.
Specifically, the measurement unit 215 may generate the spatial
distribution as a matrix of the generated measurement values such
that these are arranged to match the matrix arrangement of the
detection coils 213. Thus, specifically, the spatial processor 217
may receive the measurement values from the measurement unit 215,
e.g. in the order in which they are made, and may in response
generate a spatial distribution reflecting both the measurement
results and the positions corresponding to the measurements.
Thus, whereas the set of measurement values may comprise only the
measurement values with no spatial information, the spatial
distribution of the measurement values reflects both the
measurement values and the positions of the detection coils 213.
The spatial distribution thus reflects both the measurements and
where these were made.
The specific approach used for the spatial distribution to reflect
both measurement values and the spatial relationship between these
may depend on the individual preferences and requirements of the
individual embodiment. For example, in some embodiments, the
spatial distribution may be represented by a data structure with
the contained values corresponding to the measurement values and
the arrangement of these in the structure reflecting the spatial
arrangement. Specifically, the measurement values may be stored in
a matrix structure corresponding to the matrix positioning of the
detection coils 213. In other embodiments, the spatial distribution
may e.g. be represented by two component values in a random order
with a first component reflecting the measurement value and the
second component reflecting the position of the corresponding
detection coil 213.
It will be appreciated that although FIG. 2 illustrates different
entities performing the measurements and generating the spatial
distribution, these may in some embodiments be generated as a
combined operation. E.g. the measurement values may directly be
stored at the appropriate positions in a suitable data structure,
such as directly in a matrix data structure.
The spatial processor 217 is coupled to a foreign object detector
219 arranged to detect a presence of a foreign object in response
to the measurement spatial distribution generated by the spatial
processor 217. Specifically, the foreign object detector 219 is
arranged to compare the measurement spatial distribution to a
reference spatial distribution and to generate a detection in
response to this comparison.
E.g. if the comparison results in a similarity criterion being met,
then the foreign object detector 219 may generate an indication
that a foreign object is not detected. However, if the similarity
criterion is not met, and thus the current magnetic image differs
sufficiently from the reference magnetic image, then the foreign
object detector 219 generates an indication that a foreign object
has indeed been detected.
The foreign object detection result is fed to the power transmitter
controller 203 which may control the operation of the power
transmitter 101 in accordance with the detection result. For
example, the detection may be performed as part of the
initialization of a power transfer operation and if the detection
indicates that no foreign object is present, the system may proceed
to initialize the power transfer. However, if a foreign object is
detected then the power transmitter controller 203 may proceed to
instantly terminate the initialization such that no power transfer
is initiated.
Thus, the approach may for example prevent initialization of a
power transfer in scenarios where there may be a foreign object
present. This may e.g. increase safety precautions and e.g. ensure
that a power transfer is not initiated which could result in
undesirable heating of e.g. a coin or set of keys left in the power
transfer area.
The exact physical arrangement of the individual detection coils
213, as wells of the test signal coil 209 and the transmitter coil
103, will depend on the preferences and requirements of the
individual embodiment. Similarly, the physical dimensions and
properties of the individual coils will depend on the preferences
and requirements of the individual embodiment.
However, in many embodiments, the detection coils 213 may be
arranged in a planar array (or matrix) arrangement. Specifically,
the detection coils 213 may be arranged in an equidistant grid with
detection coils 213 being positioned at nodes of the grid within a
given detection area (in some embodiments only a subset of the
nodes may correspond to the position of a detection coil 213). An
example of such an arrangement is illustrated in FIG. 3. As can be
seen, the detection coils 213 form an area which is covered by the
detection coils 213. The detection coils 213 form a grid or matrix
and thus the corresponding measurements can be considered to
generate a magnetic image reflecting the spatial variations of the
magnetic field corresponding to the magnetic test signal.
In the example, the transmitter power coil 103 is relatively large
and defines a relatively large power transfer area. A power
receiving device 105 positioned in the relatively large power
transfer area will thus be exposed to a potentially powerful power
transfer magnetic field allowing the power receiving device 105 to
extract substantial power.
The detection coils 213 cover the power transfer area and thus can
be used to generate an effective magnetic image of the power
transfer area thereby generating a magnetic image reflecting any
objects present in the power transfer area.
In order to get a reasonable spatial resolution allowing an
efficient foreign object detection, it is often advantageous to
have a relatively high number of detection coils 213. In order, to
achieve this, the detection coils 213 may often be selected to be
relatively small, especially in comparison to the transmitter power
coil 203. This may also allow smaller objects to be detected as it
provides a higher spatial resolution.
In many embodiments, the number of detection coils 213 is typically
no less than 20 detection coils. This may in many embodiments
provide a resolution suitable for reliable object detection of
sufficiently small objects. For many practical implementations, the
detection coils 213 have a maximum dimension (typically a diameter
of a circular coil or a diagonal length of a square coil) not
exceeding 20 mm (and often in the range of 10-15 mm). Such
approaches have been found to provide particularly efficient
foreign object detection in many practical implementations, such as
e.g. for systems compatible with the Qi specifications. Further,
keeping the dimensions of the detection coils 213 sufficiently
small allows for relatively small foreign objects, such as e.g.
coins, to be detected.
In many embodiments, however, the number of detection coils 213
does not exceed 500, 200 or 100 coils. Thus, in many embodiments,
the number of measurements (and thus the number of pixels in the
magnetic image) is kept relatively low. This reduces cost and
complexity as well as facilitates the processing of the foreign
object detection. However, it has been found to still allow very
reliable foreign object detection in many embodiments.
In the specific example of FIG. 3, a total of 52 detection coils
have been used. FIG. 3 represents a top view of the power
transmitter power transfer area. The array of 52 detection coils
213 cover the power transfer area wherein the transmitter coil 103
can provide a high magnetic field strength.
FIG. 3 also illustrates the test signal coil 209. In the specific
example, the test signal coil 209 is positioned around the array of
detection coils 213, i.e. such that it encompasses the detection
coils 213. The size of the test signal coil 209 is thus in this
example larger than the combined size of the detection coils 213
and further is positioned such that it surrounds the detection
coils 213. This may provide an efficient magnetic test signal in
many embodiments.
FIG. 4 illustrates a cross-section of the power transmitter and
specifically of the arrangements of respectively the detection
coils 213, the test signal coil 209, and the transmitter coil 103.
Again, the specific arrangement may differ between embodiments but
in many scenarios the detection coils 213 may be positioned
relatively close to the surface 401 for receiving the power
receiving device 105. This may in many embodiments result in an
improved magnetic image being generated.
When measurements are being performed in order to generate the set
of measurement values, the test signal generator 211 first
generates a test signal and feeds this to the transmitter coil 103.
The exact characteristics of the test signal, and thus the
resulting magnetic test signal, will depend on the characteristics
of the individual embodiment. However, the test signal generator
211 will generate the test signal as a time varying signal
resulting in a time varying magnetic test signal which accordingly
will induce (directly or indirectly) a voltage in the detection
coils 213.
In some embodiments, the test signal may simply be generated as a
sine wave signal or a square wave signal (e.g. for a certain test
duration) which will result in a correspondingly varying magnetic
field (and specifically a varying flux density B). If no magnetic
or electrically conductive elements are present, this varying
magnetic field will induce a corresponding voltage in the detection
coils 213.
The measurement unit 215 may measure the resulting induced signal
levels, e.g. by rectifying the current and measuring the resulting
average value. This will result in a set of measurement values that
will typically reflect a fairly smooth and homogenous spatial
distribution. In other words, a homogenous magnetic image will
result.
However, if an electromagnetic element is present, this will affect
the resulting magnetic field leading to variations in the magnetic
field. The changes will have a spatial variation and effectively
the presence of the electromagnetic element will result in changes
in the measurement values for the different detection coils 213.
The electromagnetic element will effectively result in the
electromagnetic image no longer varying homogenously but rather
exhibiting spatial variations that reflect the presence of the
electromagnetic element. Indeed, in many embodiments, the
electromagnetic element may effectively leave an imprint on the
magnetic image.
For example, if a ferromagnetic or ferrimagnetic element is
positioned on the power transfer surface, and thus across one or
more of the detection coils 213, the magnetic test signal will
correspond to a magnetic field that will cause the magnetic domains
of the magnetic element being aligned with the original field
causing a magnetization of the element resulting in an increased
magnetic flux density compared to when no magnetic element is
there. Thus, the magnetic flux density through the detection coils
213 will be different than when no element is present, and
specifically a higher voltage (or emf) will be induced in detection
coils 213 proximal to the element.
A similar result may occur if a conductive, and specifically a
metallic, object is positioned on the power transfer surface
although the effect may be caused by a slightly different
mechanism. Specifically, the changes in the magnetic test signal
will result a varying magnetic flux density causing eddy currents
to be induced in the metallic object. However, these eddy currents
will then result in a magnetic field (i.e. they will result in
magnetic flux density that will combine with the one resulting from
the magnetic test signal) resulting in typically a changed magnetic
field/flux density around the object. This will also tend to result
in a different level of the induced voltage which will be reflected
by the measurements of the proximal detection coils 213.
Specifically, in the scenario, the (Eddy) currents induced in the
metallic object can cause an inverse field resulting in a decreased
magnetic field near the object. Such areas of reduced field
strength can then be detected.
An example of a magnetic field in the presence of two metallic
objects is illustrated in FIG. 5.
The measurement values may in many embodiments be generated to be
indicative of a level of the voltage induced in the corresponding
detection coils 213 by the magnetic test signal (directly or
indirectly, i.e. including the induction due to the magnetic flux
density variations caused by eddy currents induced by the magnetic
test signal). In some embodiments, the measurement values may be
generated e.g. as low pass filtered (averaged) level values. For
example, each of the detection coils 213 may be connected to a
rectifier circuit followed by a low pass filter and the output of
the low pass filters may be measured, e.g. after as suitable delay
following the start of the generation of the test signal. As
another example, the measurement may in some embodiments include a
peak detector and the measurement value may be a peak value of the
induced voltage during a time when the magnetic test signal is
generated.
As yet another example, the measurement values may be generated by
measuring the value of the induced voltage at a specific point in
time, and specifically a sampling of the voltage of the detection
coils 213 may be used. In such examples, the timing of the
measurements/sampling may be carefully controlled, e.g. by setting
a phase offset or time offset between the test signal and a sample
time control signal. Such an approach may reflect that the magnetic
mechanisms resulting in the changed behavior results in phase
changes from the original signal (e.g. induction of voltages is
dependent on the derivative of the magnetic flux density thus
introducing a phase shift for sine wave signal components). Indeed,
in some embodiments, the measurement values may reflect a phase
difference between the test signal/magnetic test signal and the
induced signals. Due to the mechanisms described above, this phase
is different for no foreign object being present and for e.g. a
metallic object being present (as the process of inducing eddy
currents resulting in additional magnetic flux contributions also
introduces additional phase shifts).
Thus, the measurement values will tend to be dependent on whether a
conductive object, such as e.g. a key or a set of keys, is present
or not. Further, as the effect tends to be localized, the spatial
evaluation by the system can provide additional information that
can be used in the foreign object. Thus, rather than merely
considering the size of a potential power loss or a quality (Q)
degradation, the system also considers the spatial information
which allows a much more accurate detection.
Indeed, the comparison to determine whether a foreign object is
considered to be present or not is based on considering the spatial
distribution of the measurement values, i.e. the detection
considers not only the measurement values but also the associated
positions of these values.
The foreign object is determined based on a comparison of the
spatial distribution of measurement values, referred to as the
measurement spatial distribution, to a reference spatial
distribution. The foreign object detector 219 thus compares the
actual measurement results to a reference and may e.g. generate a
detection of a foreign object if these differ too much, i.e. if the
measurement spatial distribution and the reference spatial
distribution fail to meet a given similarity criterion.
The foreign object detection may specifically be based on comparing
the measurement spatial distribution to a reference spatial
distribution which reflects a scenario in which no foreign object
is present, and indeed in some situations may reflect a scenario in
which no power receiving device 105 is present either.
For example, the reference spatial distribution may be generated to
consist of reference measurement values which may correspond to
values that may be measured with no objects present. These values
will depend on the specific characteristics of the generated test
signal as well as on characteristics of the test signal coil 209
and the detection coils 213, including the geometric relationships
between these. In some embodiments, such a reference spatial
distribution, or image, can indeed be generated by analytical
evaluation of the physical properties of the power transmitter
101.
In such a case, the reference spatial distribution will tend to
have relatively smooth and small variations. Indeed, in many
situations, the reference spatial distribution may correspond
almost to a spatially constant distribution, i.e. the measurement
values may be substantially the same for all detection coils 213.
In other embodiments, the magnetic flux density of the magnetic
test signal may vary somewhat over the area spanned by the
detection coils 213 due to the characteristics of the test signal
coil 209 and detection coils 213 arrangements (e.g. in the example
of FIG. 3, it may be slightly lower for detection coils 213 in the
center of the test signal coil 209 than for detection coils 213
towards the periphery of the test signal coil 209). Thus, in such
scenarios, the reference spatial distribution may contain reference
measurement values which are e.g. slightly lower in the center of
the test signal coil 209 and then gently decreasing towards the
test signal coil 209.
In such a case, the foreign object detector 219 may e.g. perform a
comparison by determining the difference between the actual
measurement value for each detection coil 213 and the corresponding
reference measurement value (e.g. after a scaling to compensate for
different average values in the distributions). If no foreign
objects are currently present, this should result in a smooth
spatial variation of differences which are furthermore likely to be
small, and ideally zero. However, if a subset, or specifically one,
of the measurement values results in a high difference measure,
this may indicate that a foreign object is indeed present. The
spatial distribution may further be considered. For example, the
difference values may be determined and all detection coils 213 for
which the difference exceeds a threshold may be identified. It may
then be evaluated whether these are part of one or more areas that
are sufficiently small to be likely to correspond to a(n
undetected) foreign object. If so, the foreign object detector 219
may determine that a foreign object is present and may generate a
detection result indicative of a foreign object being present.
In some embodiments, the reference spatial distribution may reflect
a scenario wherein a power receiving device 105 is present.
Specifically, the reference spatial distribution may reflect a
situation wherein a power receiving device 105 is present on the
power transfer surface. The reference spatial distribution may for
example include a pattern of measurements reflecting the magnetic
shape or imprint of the power receiving device 105.
For example, the power receiving device 105 may be a device which
comprises some metallic elements and accordingly the magnetic test
signal will result in measurement values which are higher for
detection coils 213 close to these metallic elements. Accordingly,
the power receiving device 105 may result in a certain pattern of
measurement values with above average value. In some embodiments,
the reference spatial distribution may reflect the magnetic pattern
of the power receiving device 105 and thus when comparing the
measurement spatial distribution to the reference spatial
distribution, this comparison may include a consideration or
expectation that a pattern of measurements with higher values will
be present.
As a specific example, the reference spatial distribution may
comprise reference measurement values which would be (expected to
be) measured by the detection coils 213 when a power receiving
device 105 is present. If the measurement spatial distribution is
subsequently generated when the power receiving device 105 is
actually present, the comparison to the reference spatial
distribution will compensate for this and a false detection of a
foreign object due to the presence of the power receiving device
105 can be avoided. Specifically, if difference values are
generated, these will also be low for the detection coils 213 for
which increased values are measured due to the power receiving
device 105 being present.
The comparison of the measurement spatial distribution and
reference spatial distribution may include an alignment of the
measurement spatial distribution and the reference spatial
distribution, i.e. the measurement and reference magnetic images
may be aligned before (or as part of) being compared to each
other.
For example, the two spatial distributions may be shifted with
respect to each other prior to the difference values being
generated. This approach may be repeated for a number of different
offsets, including for different directions and values. The
alignment which results in the lowest overall difference between
the measurement spatial distribution and the reference spatial
distribution may then be selected, and the foreign object detection
may be based on the difference values for this alignment.
As another example, the two spatial distributions may be rotated
with respect to each other prior to the difference values being
generated. This approach may be repeated for a number of different
rotations and the rotation alignment that results in the lowest
overall difference between the measurement spatial distribution and
the reference spatial distribution may then be selected, and the
foreign object detection may be based on the difference values for
this alignment.
In many embodiments, both rotations and shifts may be considered.
For example, for each offset or shift, a number of difference
values may be determined for different rotations. The lowest
difference value may be identified and used as the difference value
for that offset. This may be repeated for a number of different
offsets, and subsequently the offset resulting in the lowest
difference value may be selected and used for the foreign object
detection.
In some embodiments, additional or alternative geometric
transformations may be considered. For example, in some
embodiments, a mirroring of one of the spatial distributions may be
applied (often together with rotations and translations). This may
for example be appropriate in embodiments wherein a power receiver
may have an asymmetric shape and may e.g. also be positioned upside
down on the power transmitter. In such examples, the power receiver
may have mirror electromagnetic footprints depending on which way
up the device is positioned.
In many embodiments, the foreign object detector 219 may be
arranged to apply a geometric transformation to either the
measurement spatial distribution or the reference spatial
distribution (or both) before (or while) comparing these to each
other. For simplicity and brevity, the following example will focus
on the geometric transformation being applied to the measurement
spatial distribution.
The geomtric transformation may be expressed as a mathematical
function which transforms coordinates of the measurement spatial
distribution into new coordinates with the comparison being based
on these new coordinates. The geometric transformation may be
dependent on a number of parameters.
For example, the measurement spatial distribution may comprise
measurements for the spatially distributed detection coils where
each measurement is associated with a two dimensional coordinate.
Similarly, each value of the reference spatial distribution is
associated with a two dimensional coordinate. The comparison may
involve dividing the values of the measurement spatial distribution
and the reference spatial distribution into pairs and determining
the difference between the values for each pair. The detection may
then be performed based on these difference values. For example, an
overall difference value may be calculated as e.g. the sum or
average of the pair difference values.
The pairing may often be based on a requirement that the geomtric
distance between the coordinates of the values being paired is
minimized. However, rather than determining the distance between
coordinates of the original measurement spatial distribution and
reference spatial distribution, the coordinates of (e.g. the
measurement spatial distribution) may be subjected to a geometric
transformation before (or as part of) the distance assessment. The
geometric transformation may then be determined to minimize the
overall difference value.
The geometric transformation may for example be a function
performing a shift by a value a followed by a rotation by a given
angle .phi. (e.g. using the well-known sine and cosine rotation
functions). The values .alpha., .phi. may then be determined to
result in a minimised difference value (e.g. determined by pairing
values dependent on the transformed parameters where the
transformation is given by .alpha. and .phi.) and this may be used
to perform the object detection.
It will be appreciated that whereas the determination of the
parameters of the geometric transformation may e.g. be performed as
a post-processing applied to the measurement spatial distribution
in order to minimize the difference value, many other options are
possible. For example, in some embodiments, a control loop may be
implemented which controls the values of the geometric
transformation parameter, e.g. using the calculated difference
value as an error signal.
In some embodiments, the reference spatial distribution may be a
partial spatial distribution reflecting only a subset of the area
covered by the detection coils 213. For example, the reference
spatial distribution may only comprise a representation
corresponding to the power receiving device 105, and may thus
specifically reflect the magnetic pattern of the power receiving
device 105. The foreign object detector 219 may align this pattern
with the measurement spatial distribution by identifying a relative
position of the pattern resulting in the best match between the
pattern and the measurement spatial distribution. The pattern may
then be subtracted from the corresponding measurement values to
generate a compensated spatial distribution in which the effect of
a power receiving device 105 being present has been compensated.
The resulting compensated spatial distribution may then be
evaluated to detect if any further areas of different measurement
values can be found. Alternatively, the foreign object detector 219
may in some embodiments simply ignore the measurement values that
are considered to be within the area covered by the power receiving
device 105 (as indicated by the pattern matching).
A particular advantageous aspect of the described approach is that
the foreign object detection is not merely based on a general or
combined evaluation of whether any objects are present or not but
rather allows for a more sophisticated approach to be used which
specifically may allow a differentiation between the presence of a
power receiving device 105 and the (additional or alternative)
presence of the foreign object. Thus, the approach may enable the
differentiation between e.g. metal of the power receiving device
105 and a foreign metal object. Furthermore, the approach may allow
this differentiation to have high reliability thereby providing a
reliable foreign object detection.
For example, the reference spatial distribution may be generated to
represent a magnetic image of the power transfer surface in the
presence of a power receiving device 105. The reference spatial
distribution may accordingly include a
"profile"/"signature"/"fingerprint" of the power receiving device
105. Accordingly, the power transmitter 101 can compare this
scenario to the current situation and estimate whether a foreign
object is present or not. For example, it may consider that if at
least one of the pixels differs sufficiently, then a foreign metal
object is detected.
As an example, a cordless kitchen system based on inductive
wireless powering of appliances, may run the risk of
unintentionally heating foreign metal objects that are
inadvertently placed in the generated magnetic field for
transferring power. Therefore, it is desirable for the system to
detect the presence of such objects and e.g. to shut down the power
transfer in response. A complication is that the appliance that is
to receive power may include metal that is e.g. intentionally used
in the appliance for heating purposes ("friendly metal"). It is
therefore desirable to be able to distinguish between foreign and
friendly metals. The described approach allows the foreign object
detector 219 to reliably make such a differentiation.
It will be appreciated that different approaches for generating the
reference spatial distribution may be used in different
embodiments. For example, in some embodiments, the measurement
values for different detection coils 213 may be estimated by
theoretical calculations or simulations.
However, in many embodiments, the foreign object detector 219 may
be arranged to store a copy of a measurement spatial distribution
and use this as a reference spatial distribution for future
comparisons. The measurement spatial distribution that is stored
may specifically be one which is made when no foreign object is
present, and specifically in many embodiments where no foreign
object is present but the power receiving device 105 is
present.
For example, a user may provide a manual input to the power
transmitter 101 after positioning the power receiving device 105 on
the power transfer surface and after ensuring that no foreign
object is present. In response to this activation, the power
transmitter 101 may proceed to generate a measurement spatial
distribution using the same approach as for foreign object
detection. The resulting measurement spatial distribution will thus
reflect the situation with only the power receiving device 105
being present, and the foreign object detector 219 may proceed to
store this measurement spatial distribution as a reference spatial
distribution. This reference spatial distribution may then
subsequently be used as a reference spatial distribution.
It will be appreciated that the power transmitter 101 may
potentially support a number of different power receiving devices
and it may accordingly be able to store a plurality of reference
spatial distributions. For example, the above described manual
approach may be performed for a number of different power receiving
devices resulting in a plurality of reference spatial distributions
being stored. When performing subsequent foreign object detections,
the foreign object detector 219 may compare the measurement spatial
distribution to all of the stored reference spatial distributions
to detect whether a close match is found.
In some embodiments, the foreign object detector 219 may
alternatively or additionally be arranged to determine the
reference spatial distribution in response to data received from
the power receiving device 105.
In most wireless power transfer approaches, the system includes
functionality for communicating data between the power receiver and
the power transmitter (and often for bilateral communication). For
example, the Qi specification allows for the power receiver to
communicate data to the power transmitter by load modulating the
power transfer signal. Alternatively or additionally, communication
may be supported by dedicated communication functionality using
e.g. separate communication coils. For example, in some scenarios
the power transmitter and power receiver may communicate with each
other using out-of-band communication, and may specifically use
dedicated short range communication approaches.
FIG. 6 illustrates the system of FIG. 1 but further including such
dedicated communication functionality. Specifically, the power
transmitter 101 comprises a dedicated communication coil 601 and a
communication unit 603. Similarly, the power receiving device 105
comprises a dedicated communication coil 605 and a communication
unit 607. The two communication units 603, 607 can communicate data
via signals exchanged using the two communication coils 601, 605.
The communication may specifically be implemented using a standard
short range communication signal, such as for example Bluetooth or
Near Field Communication.
The power receiving device 105 may in such embodiments transmit
data to the power transmitter 101 which allows this to generate the
reference spatial distribution in response. In some embodiments,
the power receiving device 105 may explicitly communicate data
fully describing the reference spatial distribution. In other
embodiments, the power receiving device 105 may communicate e.g.
identification data allowing the power transmitter 101 to select a
reference spatial distribution from e.g. a plurality of
predetermined reference spatial distributions. Such predetermined
reference distributions may e.g. be stored locally, or may e.g. be
stored in a central server.
For example, whenever a new type of power receiving device is
developed, the reference spatial distribution may be determined and
by the manufacturer submitted to a central server which e.g. may be
accessible via the Internet. The reference spatial distribution may
be stored with an associated identity. When the power receiving
device 105 subsequently is to be used with a power transmitter 101,
it may communicate the identity to this. In response, the power
transmitter 101 may access the central server using the received
identity and may in this way retrieve a suitable reference spatial
distribution to use for foreign object detection.
As another example, the power receiving device 105 may communicate
a locally stored reference spatial distribution to the power
transmitter whenever it detects the presence of a power transmitter
101 (or e.g. as part of the initialization of a power transfer
operation).
Such approaches may be more efficient in some embodiments.
Specifically, it may obviate the need for the power transmitter to
locally store reference spatial distributions for all possible
power receiving devices. It may further effectively support foreign
object detection for new power receiving devices being developed
and introduced to the market.
Thus, it will be appreciated that various approaches can be used
for implementing the physical location to store the reference
spatial distribution. A first option is to store it at the power
transmitter. A second option is to store it at the power receiver.
A third option is to store it at a storage service outside of the
power transfer system (e.g. a server reachable via the Internet).
It will also be appreciated that all of these options can be used
independently of where the reference spatial distribution is
generated, i.e. whether the reference spatial distribution is
generated by the power transmitter, by a manufacturer of the power
receiving device, etc. It will also be appreciated that
combinations of such approaches may be used (e.g. with a central
server providing most reference spatial distributions but with the
power transmitter locally generating and storing reference spatial
distributions for power receiving devices not represented in the
central server).
As previously described, the comparison of the measurement spatial
distribution and the reference spatial distribution may generally
include a geometric alignment of the measurement spatial
distribution and the reference spatial distribution. This alignment
may be performed by applying a geometric transformation to either
the measurement spatial distribution or the reference spatial
distribution (or to both). The geometric transformation may include
a translation and/or a rotation. In many embodiments, the geometric
transformation may be determined by minimizing the difference
between the measurement spatial distribution and the reference
spatial distribution using a suitable minimization criterion. For
example, the transformation resulting in the lowest square
differences may be determined and applied in the comparison.
In some embodiments, the power transmitter 101 may furthermore
comprise a user output unit, such as e.g. a display. In such cases,
a user output may be generated on the basis of the geometric
transformation.
For example, the reference spatial distribution may be generated to
correspond to an optimal power transfer position for the power
receiving device 105. As part of the comparison, the geometric
transformation may be determined. The geometric transformation may
specifically reflect a vector which the reference spatial
distribution and measurement spatial distribution must be shifted
relative to each in order to minimize the difference between them
(e.g. assuming that the power receiver magnetic image pattern is
rotationally invariant). However, in such cases, the vector will
also provide a good indication of the offset between the current
position of the power receiving device 105 and the optimal position
of the power receiving device 105 (from a power transfer point of
view). Accordingly, the foreign object detector 219 may control the
output display to indicate a vector indicating how the power
receiving device 105 should be moved to achieve optimum power
transfer.
In the previous examples, the foreign object detection has been
based on only a single measurement spatial distribution (and
reference spatial distribution). However, it will be appreciated
that in other embodiments, the detection may be based on a
plurality of sets of measurements.
Specifically, in some embodiments the power transmitter 101 may be
arranged to perform the described operation for a plurality of
different frequencies of the test signal. Specifically, the test
signal may be a periodic signal which is active for a certain time
duration. During this interval, the test signal thus has a given
repetition frequency. For example, during the test time interval, a
sine wave or square wave signal may be generated and the resulting
induced voltages in the detection coils 213 measured.
In some embodiments, this approach may be repeated for the test
signal (and thus the magnetic test signal) having different
frequencies. For each frequency, a measurement spatial distribution
may be generated and compared to a reference spatial distribution.
The foreign object detection may be based on a plurality of these
comparisons. For example, for each frequency, a difference measure
may be generated indicating the difference between the measurement
spatial distribution and the reference spatial distribution. A
combined difference measure may then be determined, e.g. by
averaging or summing the individual difference measures. If this
combined difference measure exceeds a threshold, the foreign object
detector 219 may indicate that a foreign object has been detected
and otherwise it will indicate that no foreign object has been
detected.
Thus, in some embodiments, the test signal generator may be
arranged to generate a plurality of test signals with different
frequencies resulting in a plurality of magnetic test signals with
different frequencies. For each of these test signals, the
measurement unit may generate a set of measurement values thereby
generating a plurality of sets of measurement values. For each set
of measurement values, a measurement spatial distribution may
subsequently be generated.
The foreign object detector may then determine the foreign object
detection estimate based on a plurality of comparisons, each
comparison being between a measurement spatial distribution of the
plurality of measurement spatial distributions and a reference
spatial distribution. The reference spatial distributions may in
many embodiments vary for different frequencies thereby reflecting
the variations in the reference scenario due to varying
frequencies.
Such an approach may in many embodiments and scenarios provide
improved performance. In particular, the influence of metal objects
on the magnetic field may typically depend on the frequency of the
field (and thus of the test signal). Accordingly, making additional
measurements at different frequencies provides additional
information that may result in an improved detection
performance.
In some embodiments, the measurement unit 215 may be arranged to
perform the measurements of the induced voltages simultaneous for
all of the detection coils 213. Thus, a parallel measurement
operation may be performed.
However, the approach does not rely on or require that the
measurements are necessarily simultaneous. Rather, in some
embodiments the measurement unit 215 may generate the set of
measurement values by sequentially measuring induced signals of the
plurality of spatially distributed detection coils. The sequential
measuring comprises separately measuring subsets of the plurality
of spatially distributed detection coils. For example, the set of
measurement values may be generated by the same measurement circuit
sequentially, one-by-one, separately measuring the voltage of a
detection coil 213. In some embodiments, a plurality of parallel
measurement units may be used and thus the sequential measurement
may include e.g. two detection coils 213 in each iteration.
This may in many embodiments allowed reduced complexity of the
power transmitter 101 thereby resulting in reduced cost. In
particular, it may in many embodiments reduce the amount of
measurement hardware required.
It will be appreciated that the above description for clarity has
described embodiments of the invention with reference to different
functional circuits, units and processors. However, it will be
apparent that any suitable distribution of functionality between
different functional circuits, units or processors may be used
without detracting from the invention. For example, functionality
illustrated to be performed by separate processors or controllers
may be performed by the same processor or controllers. Hence,
references to specific functional units or circuits are only to be
seen as references to suitable means for providing the described
functionality rather than indicative of a strict logical or
physical structure or organization.
The invention can be implemented in any suitable form including
hardware, software, firmware or any combination of these. The
invention may optionally be implemented at least partly as computer
software running on one or more data processors and/or digital
signal processors. The elements and components of an embodiment of
the invention may be physically, functionally and logically
implemented in any suitable way. Indeed the functionality may be
implemented in a single unit, in a plurality of units or as part of
other functional units. As such, the invention may be implemented
in a single unit or may be physically and functionally distributed
between different units, circuits and processors.
Although the present invention has been described in connection
with some embodiments, it is not intended to be limited to the
specific form set forth herein. Rather, the scope of the present
invention is limited only by the accompanying claims. Additionally,
although a feature may appear to be described in connection with
particular embodiments, one skilled in the art would recognize that
various features of the described embodiments may be combined in
accordance with the invention. In the claims, the term comprising
does not exclude the presence of other elements or steps.
Furthermore, although individually listed, a plurality of means,
elements, circuits or method steps may be implemented by e.g. a
single circuit, unit or processor. Additionally, although
individual features may be included in different claims, these may
possibly be advantageously combined, and the inclusion in different
claims does not imply that a combination of features is not
feasible and/or advantageous. Also the inclusion of a feature in
one category of claims does not imply a limitation to this category
but rather indicates that the feature is equally applicable to
other claim categories as appropriate. Furthermore, the order of
features in the claims do not imply any specific order in which the
features must be worked and in particular the order of individual
steps in a method claim does not imply that the steps must be
performed in this order. Rather, the steps may be performed in any
suitable order. In addition, singular references do not exclude a
plurality. Thus references to "a", "an", "first", "second" etc. do
not preclude a plurality. Reference signs in the claims are
provided merely as a clarifying example shall not be construed as
limiting the scope of the claims in any way.
* * * * *
References